The present invention is directed to the silicon carbide cutting inserts of the type produced in accordance with teachings of U.S. patent application Ser. Nos. 167,019 and 167,196, both filed July 9, 1980, both abandoned in favor of co-pending continuation application Ser. Nos. 313,241, now U.S. Pat. No. 4,428,755, and 312,987, now U.S. Pat. No. 4,417,906, respectively, both by Dr. John M. Ohno, and assigned to the assignee of the present invention, the entire disclosures of which are hereby incorporated by reference.
The above-mentioned patents disclose a straight-forward technique (hereinafter referred to as the "press and treat" technique) for forming high quality cutting inserts, which readily lends itself to mass production techniques. Very briefly, the press and treat technique involves the preparation of a first or crystal dispersion of super-hard crystals such as diamond or cubic boron nitride crystals in carbon black and a second or core dispersion of carbon black, carbon fiber and filler material (super-fine .beta.-silicon carbide). The two dispersions are individually mixed with a small amount of temporary binder such as paraffin to lend a sufficient green strength to the two dispersions upon cold compaction thereof. After compacting the two dispersions together in a desired configuration, the compact is vacuum heated in the presence of silicon to burn off the paraffin and to allow the silicon to infiltrate both dispersions. Upon further heating, and without the need for the constant application of any type of pressure to the insert, the silicon reacts with the carbon black to form a .beta.-silicon carbide and silicon matrix which bonds both dispersions both internally and to each other.
The dispersions may be compacted in a number of ways, several of which are disclosed in co-pending U.S. patent application Ser. Nos. 226,604, now abandoned; 286,613; 331,381, now U.S. Pat. No. 4,448,591, the entire disclosures of which are hereby incorporated by reference, as well as the above-mentioned pending patent applications. For example, the Ser. No. 167,196, now abandoned, the two dispersions are compacted at the same time to provide a first dispersion which completely overlies the second dispersion. In Ser. No. 167,019, now abandoned, on the other hand, a two step compaction technique is employed such that the super-hard crystal dispersion is only disposed along the periphery of the lightly prepressed core. In Ser. Nos. 226,604 now abandoned, and 286,613, multi-step processes are employed to provide particular configurations for the inserts. None of the above compaction techniques are employed to provide any type of warpage compensation.
In most applications it is necessary that the insert produced by the press and treat technique be mounted upon a carbide substrate, such as tungsten carbide in order to lend strength to the cutting insert. The applications include high speed machining, or interrupted machining of Al-Si alloy, Cu alloy, ceramic, fiberglass reinforced plastics, graphite, pre-sintered carbide, etc. It is therefore of paramount importance that the inserts so produced have a high degree of flatness on their lower surfaces in order to provide a secure bond to the carbide substrate.
It has been found, however, that for a wafer typ composite, the amount of warpage of the lower surface during sintering renders the bonding of the wafer to the carbide substrate difficult or inadequate regardless of the bonding technique.